A great many methods and apparatus for mixture formation using an oxygen sensor are already known. Frequently, oxygen sensors are utilized which change their output quantity abruptly at a (.lambda.=1) mixture. In the closed control circuit, the oxygen sensor normally always oscillates between the two output quantities "high" and "low".
The oxygen sensor output signal conventionally serves to correct the values stored in a fixed pre-programmed set of characteristic curves, these values determining, for example, the start of injection. On the basis of the Lambda sensor signal which is to be regarded as a quasi-binary signal, the correction factor for the set of characteristics is continuously corrected by a PI-controller, for example.
Since, in this arrangement, the air transit time through the internal combustion engine and the sensor response time are not taken into consideration, a limit cycle will occur for the correction factor of the Lambda regulator and thus, of course, also for the torque of the internal combustion engine. In particular, at low engine speeds and sufficiently high load, the operator of the vehicle equipped with the internal combustion engine will become aware of this torque limit cycle which manifests itself unpleasantly in the form of an uneven running condition. In addition, with the falling frequency, exhaust emissions will rise.